a) Field of the Invention
The invention relates to a spread spectrum communication method and a communication device adopting this method, and more particularly to a frequency hopping type spread spectrum communication method and device.
b) Description of the Related Art
A spread spectrum communication is a communication method to transmit a signal with its bandwidth spread over a bandwidth wider than the frequency bandwidth of data to be transmitted, and has advantages that it is resistive to interference, can keep signals in secrecy, and can accomplish high resolution distance measuring. In addition to fields of satellite communications, ground communications, or the like, a spread spectrum communication, is being applied to mobile and local communications with the expectation of improving frequency utilization efficiency while maintaining compatability with existing systems.
Typical methods for achieving spread spectrum communication include DS (Direct Sequence) and FH (Frequency Hopping) methods. A DS method spreads an occupied frequency bandwidth by balanced modulation of a direct spread code pulse to data modulated by a carrier wave, while an FH method utilizes a broad occupied frequency bandwidth by switching (or hopping) the carrier frequency of the modulated data according to a spread code pulse. Especially, a fast FH method switches the frequency faster than the information rate so as to be resistive against interference and to excel in distance and frequency diversity effect. This fast FH method is being marked for mobile communications and indoor communications which are heavily affected by fading.
Generally, the FH method modulates data by an FSK (Frequency Shift Keying). Specifically, data to be transmitted is converted into a codeword for every several bits, and a frequency is shifted according to the codes (codeword chip) forming the codeword. For example, data is converted into one of eight codewords for every 3 bits of input data. More specifically, when input data is "000", it is converted into a codeword "7-6-5-2-4-1-3". "0" to "1" which form the codeword are simply referred to as a code or codeword chip, and it is devised to array the codes in each codeword so that data of "000" to "111", can be classified on a receiving side.
A different frequency is allocated to the respective codes. For example, frequencies f0 to f7 correspond to codes "0" to "1", respectively. To transmit "000", according to its corresponding codeword "7-6-5-2-4-1-3", the frequencies are changed into order of "f7, f6, f5, f2, f4, f1, f3" and outputted. Since the eight types of frequencies f0 to f7 are used, modulation in this case can be 8-level MFSK (Multilevel FSK) modulation. Data modulation (modulation regardless of the frequency hopping) will be referred to as primary modulation.
The frequency of a carrier wave is hopped according to a diffusion code series (pulse train of pseudo noise code; hereinafter referred to as "code series") for frequency hopping modulation. If the number of codes contained in this code series is 31, 31 different frequencies are selected as hopping frequencies in a frequency band approved for use (the frequency hopping itself may also be said to be FSK in a broad sense, but the term FSK in this specification is used for the primary modulation only). A cycle in which the code series takes a round is referred to as a code cycle, and a cycle (1/31 of the code cycle in this case) in which the hopping frequency is switched is referred to as a hopping period. The hopping frequency is switched in synchronization with the switching of a frequency by the primary modulation.
Such an FH-MFSK method is introduced as a combination having higher affinity than the existing methods in various publications, one example being "Frequency-Hopped Multilevel FSK for Mobile Radio" by D. J. Goodman, et. al., The Bell System Technical Journal, Vol. 59, No. 7, pp. 1257-1275, 1980. A combined method of M-ary FSK which is an improved version of MFSK and FH is reported in "General Description and Basic Characteristics of Ground Mobile Frequency Hopping Type Communication Experimental Arrangement" by Eimatsu Moriyama, et. al., Quarterly Journal of Radio Research Laboratory, Vol. 32, No. 164, pp. 165-177, 1986.
A conventional MFSK and FH combined method will be described. FIG. 4 shows a conceptual diagram illustrating the allocation of carrier frequencies by the conventional MFSK+FH method.
FIG. 4 shows that 6-bit digital data is first converted into a combined pattern of seven frequencies by primary modulation. These seven frequencies have frequency patterns selected according to the 6-bit digital data, and a single carrier frequency is selected in respective time slots (T1, T2, . . . , T7). A table showing which frequency (f1, f2, . . . , f8) is selected in the respective time slots (T1 to T7) is referred to as a primary modulation matrix through this specification.
In the case of the primary modulation matrix indicated by (a) in FIG. 4, the frequency f2 is selected in time slot T1, and frequency f6 is selected in the time slot T2. Thus, the 6-bit digital data is, so to speak, coded into a combined pattern of seven frequencies by the primary modulation matrix.
Then, frequency conversion, namely spread modulation, is performed for frequency hopping (FH). This frequency hopping is performed by further spreading the frequencies f1 to f8 which were selected by the primary modulation matrix to 127 types of carrier frequencies. In this specification, a table showing diffusion is referred to as a hopping matrix. In the hopping matrix shown in FIG. 4, the horizontal axis indicates the time slots T1 to T7, while the vertical axis indicates 127 types of frequency carrier waves. These frequencies F1 to F127 are indicated with an uppercase "F" added to distinguish them from the intermediate frequencies f1 to f8.
For example, in the case shown in FIG. 4, frequency f2 is selected in time slot T1 by primary modulation. This frequency f2 is converted into frequency F4 by the hopping matrix shown in FIG. 4. The frequencies f1 to f8 undergone the primary modulation are converted into the frequencies F3 to F10 in time slot T1 as shown in the hopping matrix. Accordingly, eight boxes indicated by a heavy line in the hopping matrix correspond to the eight frequencies f1 to f8 in a single time slot of the primary modulation matrix.
Diffusion of frequencies by the hopping matrix is determined by which hopping matrix frequency (F1 to F127) the frequency f1 in a predetermined time slot is to be converted. For example, by the frequency hopping indicated by the hopping matrix shown in FIG. 4, the frequency f1 is converted into the frequency F3 in the time slot T1, F100 in the time slot T2, and F55 in the time slot T3.
As described above, the communication method, in that the primary modulation is performed by the MFSK modulation using a plurality of frequencies and the frequency hopping (diffusion modulation) is performed by the diffusion code series, uses a signal with only a single frequency in a single time slot. Therefore, improvement of the transmission efficiency is limited.
The transmission efficiency will be described with reference to the primary modulation matrix shown in FIG. 4.
By the conventional method shown in FIG. 4, the eight types of frequencies f1 to f8 can be selected in each time slot. Since these eight frequencies are selected over seven time slots, the total number of possible patterns is 8.sup.7 (=2,097,152). Meanwhile, since 6-bit digital data is converted into a pattern formed of these seven frequencies, the number of actually used patterns is 2.sup.6 (=64). Therefore, the total number of patterns is related to the number of used patterns and expressed by the following expression. EQU Total number of patterns/Number of used patterns=8.sup.7 /2.sup.6 =2.sup.21 /2.sup.6 =2.sup.15 (1)
Therefore, the number of patterns to be used is only 1/2.sup.15 the total number of patterns. In other words, the pattern of selectable frequencies is given an allowance of 15 bits.
Since the conventional communication method adopting MFSK and FH outputs only one frequency in each time slot, improvement of the transmission efficiency is limited.